This is because they were either added to the Regular Cast section after they were already added to the episodes as Guest Stars or they were added as Guest Stars without being removed from the Regular Cast section. These two cases cause both the affected cast members to disappear from the Cast & Crew page.

You need to remove the affected cast from the episodes they appear in, check their profile that House of the Dragon doesn't appear, and re-add them to either the Regular Cast section or as Guest Stars to the episode they appear in, depending on how they're credited in the show.